input+lnwallet+contractcourt: define SignDetails for HTLC resolutions

lnwallet/commitment.go

@@ -235,6 +235,24 @@ func HtlcSigHashType(chanType channeldb.ChannelType) txscript.SigHashType {
 	return txscript.SigHashAll
 }
 
+// HtlcSignDetails converts the passed parameters to a SignDetails valid for
+// this channel type. For non-anchor channels this will return nil.
+func HtlcSignDetails(chanType channeldb.ChannelType, signDesc input.SignDescriptor,
+	sigHash txscript.SigHashType, peerSig input.Signature) *input.SignDetails {
+
+	// Non-anchor channels don't need sign details, as the HTLC second
+	// level cannot be altered.
+	if !chanType.HasAnchors() {
+		return nil
+	}
+
+	return &input.SignDetails{
+		SignDesc:    signDesc,
+		SigHashType: sigHash,
+		PeerSig:     peerSig,
+	}
+}
+
 // HtlcSecondLevelInputSequence dictates the sequence number we must use on the
 // input to a second level HTLC transaction.
 func HtlcSecondLevelInputSequence(chanType channeldb.ChannelType) uint32 {